The present invention relates to a process for recovering the recharge life cycle of sealed liquid electrolyte batteries. In particular, the invention relates to a method for recovering nickel-cadmium (Ni-Cd) cells which have degraded to the point where they no longer accept or are capable of holding a voltage charge. The preferred embodiment of the invention is directed to Ni-Cd batteries, but the principles of the invention are equally applicable to other sealed liquid electrolyte rechargeable batteries, such as nickel-metal-hydride, or rechargeable alkaline batteries.
All batteries contain an electrolyte of one form or another. The purpose is to provide a medium in which ions move freely from one electrode to another, thereby allowing current flow. Both the volume and strength of the electrolyte are important to the proper operation of the battery. As long as the electrolyte parameters stay within fairly wide limits, operation is not impaired, but if either the volume or strength of the electrolyte move outside of these fairly wide limits, the battery operation degrades. In a Ni-Cd battery and many other batteries, this electrolyte takes the form of a liquid solution of potassium hydroxide (KOH) which is soaked into the insulating material and in contact with the battery electrodes.
The invention is primarily concerned with rechargeable batteries of the general type which may be discharged at peak current drains up to 10-20 amps (A) but typically operate between 0.05 and 10 ampere hour (Ah) capacities. The charging of such batteries is typically accomplished at a charge rate of 0.1-1 amps. The charge and discharge rate of batteries is frequently referred to as the "C-rate," which is defined as the current flow out of or into a cell numerically equal to the cells rated capacity. For example, the C-rate of a 1-Ah cell is 1 A; the C-rate of a 2.5-Ah cell is 2.5 A. The C-rate concept is useful to enable a comparison of different size cells on the basis of a scaling of charge and discharge rates, for different cells behave similarly in terms of their voltage and operation time when they are charged or discharged at the same C-rate. The Ni-Cd batteries can operate with discharge rates up to 20 C, and are typically recharged at rates of 0.02 C-0.3 C.
A Ni-Cd battery is typically constructed of a tightly wrapped coil of materials consisting of a cadmium plate, a nickel plate and a porous insulating material separating the plates, which material is impregnated with an electrolyte solution such as KOH. The electrochemistry of a Ni-Cd cell determines its nominal operating voltage; for a Ni-Cd battery this nominal operating voltage is 1.2 volts. The cell chemistry causes the nominal voltage to "droop" during discharge, although the discharge voltage characteristics are quite flat over an extended operating time, and the "droop" becomes sudden and quite drastic after a predetermined operating time interval. After a Ni-Cd battery has been recharged, the voltage operating characteristic returns to its initial 1.2 volt nominal voltage, and the voltage operating curve is repeatable over many subsequent charge and recharge cycles. Commercially available Ni-Cd batteries are typically advertised to have an operating life of 300-500 recharge cycles.
It has long been suspected that the failure of such batteries is caused by a drying action, whereby the separator between the electrodes becomes dry as a result of loss of electrolyte. The mechanisms which cause such electrolyte loss are not as well known, although several theories have been urged to explain such loss. One such theory contends that the positive electrodes thicken with charging and recharging cycling, and positive active material moves toward the electrode plate surface and, in so doing, greatly reduces the pore size at the surface where contact is made with the separators. The capillary action of the exposed pores is thus increased, causing electrolyte to be drawn out of the separator more strongly than when the cell was new. At the same time, the expansion of the positive-plate electrode provides more void volume inside the plates where the absorbed electrolyte is trapped so that it cannot be recirculated.
Another theory holds that a certain volume of air or other gas is always included in the pores of the electrode plates when the cells are assembled, and, the volume occupied by the nitrogen in this air will eventually become occupied with electrolyte. Some fraction of this electrolyte is likely to come from the separator, thereby drying the separator material to the extent that this migration occurs.
Another and perhaps a better theory is that the charging and recharging of the battery results in the buildup of internal heat, depending to a large extent upon the magnitude of current passing between the electrodes. As the battery internal heat builds up, it increases the internal battery pressure, and this pressure exerts an outward force along all battery seals and, in particular, can be released by an internal pressure valve mechanism which is designed into the battery for that purpose. If the internal valve mechanism permits the release of gas under excess pressure, it will also permit the release of some electrolyte or electrolyte chemicals. Over a period of extended use, this amounts to a gradual reduction in the total electrolyte remaining inside the cell, which causes the drying of the separator material.
It may also be that the continued charging and discharging of the battery, and the heat and high internal pressures associated therewith, results in a buildup of gases along the interface between the battery plates and the separator material and into the surface of the battery plates, and this gaseous buildup reduces the efficiency of transfer of ions between the plates and the electrolyte-filled separator material. Under this scenario, the gaseous buildup may not necessarily be of sufficiently high pressure so as to release the internal battery pressure valve mechanism, but the accumulation may be sufficient to reduce the charging/discharging efficiency of the battery over time. As a gaseous buildup accumulates over time, the overall battery efficiency may degrade to the point where the battery is deemed to be no longer capable of accepting the charge, and is therefore discarded.
The battery recharge cycle is a particularly important factor in battery life, as the battery recharge current may cause a buildup of internal heat in the battery. Minimum commercial battery recharge rates are about C/20, i.e., requiring 20 hours to recharge a battery to its rated capacity. However, since the battery charging efficiency is under 100 percent, it is more typical to require 36-48 hours for recharging a battery to its rated capacity. Recharging efficiency decreases as the battery nears its full recharge. As the battery is recharged to within its final few percentage points of full capacity, the cell approaches an overcharge condition, and in this condition the cell generates gaseous oxygen O.sub.2. If the recharge current rate is low, a continuous overcharge does not damage the battery since the cell can electrochemically recombine the total oxygen volume. However, if the current recharge rate is high, and particularly over the recommended recharge limits, the internally-generated O.sub.2 is expelled from the battery through the internal pressure valve mechanism. Repeated occurrences of this type causes water to dry from the electrolyte, thereby reducing the overall life expectancy of the battery. Market conditions encourage battery manufacturers to recommend faster recharge rates for their Ni-Cd batteries, and it is not uncommon for a manufacturer to recommend a "standard" charging rate of 0.2 C (16-20 hours), but also to provide recommendations for a "fast" charging rate of 1.0 C or higher, which may cause venting of internally-generated O.sub.2. It is believed that commercially-available battery cells can sustain a continuous overcharge at up to about 0.33 C and still internally recombine 100 percent of the O.sub.2 generated at this recharging rate.
A commercially-available Ni-Cd battery pack typically is an assembly of a multiplicity of Ni-Cd cells, usually connected electrically in series. This provides a predetermined battery-pack output voltage which is determined by the number of series-connected cells, and the battery capacity is determined by the respective sizes of each of the cells. However, cells of the same size and manufacture do exhibit actual capacities that differ, up to .+-.10%, from a mean value. When such cells are connected into a multi-cell capacity, these variations can cause some cells to give up the last of their usable capacity and other cells to retain a certain capacity upon discharge of the battery pack. If the extent of battery-pack discharge is deep enough, one or more cells may be brought to zero voltage, which can cause the condition known as cell reversal.
When cell reversal occurs the energy of the cell is expended to the point where any further current drain is into the cell rather than from the cell, and therefore the external circuits drive the cell instead of the reverse action. During cell reversal, the cell voltage can drop as low as -1.4 volts, which generates gaseous hydrogen (H.sub.2), which does not recombine within the cell and must be vented through the cell internal pressure valve mechanism. This condition further contributes to the loss of water from the cell, and the ultimate drying of the electrolyte within the cell.
Ni-Cd batteries can further experience actual electrolyte leakage under certain conditions, particularly conditions wherein the battery is connected into a circuit where there is a very small amount of current drain over an extended period of time. For example, in a Ni-Cd battery connected into a radio which retains some small amount of current drain even when not being operated, a small amount of electrolyte may leak from the seals of the cell in an extended time period of from one week to several months. This is known as "creep" leakage, and usually can be identified upon inspection, as a white fuzz appearing around the top seal of the cell. Such a battery can usually be recharged, although the condition does contribute to the overall shortening of the battery life cycle.
Another factor which shortens the overall life of a Ni-Cd battery is the operating environment temperature. In general, for every 10.degree. C. increase in the average temperature of operation of a Ni-Cd battery, the cell life is cut in half. This is thought to be caused by a loss of electrolyte in the cell, combined with the breakdown of the separator material which absorbs the electrolyte liquid, or an accumulation of gases which interferes with the charge/discharge process.
The foregoing and other causes eventually lead to spent batteries, wherein the batteries are no longer able to accept a recharge, and the spent batteries must then be disposed of in a suitable manner. This presents a particular problem for Ni-Cd batteries, because cadmium is considered to be a hazardous waste and, therefore, such batteries cannot be lawfully disposed of in the usual municipal waste stream. The batteries must, therefore, be recycled in a manner which permits the recovery of the cadmium, which can then be disposed of according to the rules for disposing hazardous wastes. Approximately, 300,000,000 nickel cadmium cells are purchased in the United States each year, with the number increasing each year, and therefore, the disposal of such cells presents a real and significant problem. It, therefore, would be highly advantageous if such cells could be recycled in a manner which regenerates the cell charge-accepting capacity, and permits the cell to be reused for a significant number of additional charge/discharge operating cycles.